Torque transmitting systems are widely employed in automobile transmissions selectively to control, relative rotation between components of the transmission. As is well known to the art, one widely accepted form of an automatic, vehicular transmission employs compound planetary gear sets that utilize three clutch assemblies and two braking bands to preclude relative rotation between selected components in order to obtain the desired function of the compound planetary gear sets. The operator selects the driving range from the neutral, forward (either the standard drive, the "Intermediate" or the "Lo" forward selections) or reverse, and the transmission automatically changes gear ratios in relation to the vehicle speed and the engine torque input, as permitted within the range selected. Vehicle speed and engine torque signals are constantly fed to the transmission in a manner well known to the art in order to provide the proper gear ratio for maximum efficiency and performance at all throttle openings.
A planetary gear train consists of a center, or sun, gear, an internal gear and a planetary carrier assembly which includes and supports the smaller planet gears, or pinions. When the sun gear is held stationary and power is applied to the internal gear, the planetary gears rotate in response to the power applied to the internal gear and thus "walk" circumferentially about the fixed sun gear to effect rotation of the carrier assembly in the same direction as the direction in which the internal gear is being rotated.
When any two members of the planetary gear train rotate in the same direction and at the same speed, the third member is forced to turn at the same speed. For example, when the sun gear and the internal gear rotate in the same direction, and at the same speed, the planet gears do not rotate about their own axes but rather act as wedges to lock the entire unit together to effect what is known as direct drive.
Whenever the carrier assembly is restrained from spinning freely, and power is applied to either the sun gear or the internal gear, the planet gears act as idlers. In that way the driven member is rotated in the opposite direction as the drive member. Thus, when the reverse drive range is selected, a brake band assembly may be actuated frictionally to engage the carrier assembly, and restrain it against rotation, so that torque applied to the sun gear will turn the internal gear in the opposite direction in order to reverse the rotational direction of the drive wheels, and thereby reverse the direction of the vehicle itself. The friction band assemblies are normally operated by servo mechanisms, many varieties of which are known to the art, but the present invention does not relate servo mechanisms, and they will not be further described herein.
It should be appreciated that a second friction applying band assembly may also be employed when the engine compression, acting through the transmission, is being employed to effect a braking action. To understand this operation it is desirable to know that in a compound planetary gear set, multiple planetary gear sets may be employed, and adjacent planetary gear sets may utilize sun gears fabricated in one piece. A sprag assembly is frequently employed selectively to preclude the common sun gears from rotating in one direction.
Adjacent planetary gear sets also generally connect the carrier of the first set to the internal gear of the second set. To make the two planetary gear sets effective a roller clutch assembly is generally employed to restrain the carrier of the second set against rotation in at least one direction.
To provide a means of connecting and disconnecting the power output from a torque converter to the transmission gear train, a clutch assembly is generally employed. Typically, a clutch assembly includes a clutch housing which is splined to the input shaft. A series of torque plates are connected, as by tangs, to the clutch housing, and a second series of torque plates are connected, also by tangs, to a clutch hub member. An actuating piston is hydraulically operated frictionally to lock the torque plates together, and a release spring is employed to retract the piston when the hydraulic pressure is released. By effecting a spline connection between the main transmission shaft and the clutch hub member, whenever hydraulic pressure is supplied to the clutch assembly the input shaft directly rotates the main transmission shaft. When the hydraulic pressure is released, the clutch assembly disengages the aforesaid drive connection, and the transmission is in neutral.
A similar clutch arrangement may also be employed selectively to connect the outer race of a sprag to the transmission housing. When the outer race of the sprag is so connected to the housing, the sprag is effective in securing the sun gear connected to the sprag against rotation, and the power output from the torque converter is received by the transmission output shaft at the gear reduction ratio associated with "second" gear.
A third such clutch arrangement is generally employed to lock the pinions of the adjacent planetary gear set together so that they act as wedges to allow the two adjacent planetary gear sets to rotate as one unit. In this arrangement the power output from the torque converter is received by the transmission output shaft in what is designated as "third" gear.
Actuation of the first and third described clutch arrangements is generally effected when the operator selects the "reverse" range of operation.
Accordingly, it is apparent that the piston assembly by which the aforesaid clutch arrangements are actuated are of significant importance to the operation of such a transmission. However, under current practice the actuating piston reciprocates within a finely machined piston chamber provided within the housing of the transmission in order that the seals carried on each piston can cooperate with those finely machined surfaces to effect the requisite sealing effect along the piston during its full stroke.
Transmission cases have historically been, and continue to be, metallic castings. Castings do, on occasion, incorporate voids, but even microscopic voids, which are likely to be considered as determining the porosity of the casting, can be adversely located, and can be of such localized abundance, that when the casting is machined one or more of the machined surfaces will prove to be unacceptable for the intended purpose of those surfaces. As should be readily apparent, the structural properties of strength and hardness required to make an acceptable transmission case are not necessarily conducive to providing a readily machinable casting. In fact, variations in the section thickness of a casting, and particularly a casting having the complexity of a transmission case, can cause localized hard, or soft, spots. Although one might ideally desire a more homogenous casting, such localized variations in the physical properties of the casting may not themselves negate the suitability thereof to serve as a transmission case. Unfortunately, however, such localized variations can adversely affect the ability of the casting to be acceptably machined.
For example, the reaction of a machine tool against a localized hard spot can cause microscopic, if not macroscopic, grooves which could well preclude the effective sealing of the pressure chamber required between the prior known actuating piston and the chamber formed within the transmission housing, and such flaws might not be identified until after at least partial assembly of the transmission. But even if the flaw were to be detected at the earliest possible opportunity, the result would be a rejected transmission case.
In today's highly automated production facilities the rejection of components, and particularly major components, can not only severely increase the cycle time required to deliver that component to the assembly line but can also significantly increase the cost of that particular component.
Inasmuch as three actuating piston assemblies are normally employed in a typical automatic transmission which employs compound planetary gear sets, it becomes rather apparent that the potential for possible rejection as the result of machining the cast transmission housing is magnified by the prior known actuating piston assemblies.